Serotoninergic neurotransmission is implicated in MDD, as demonstrated by the relative success of selective serotonin reuptake inhibitors (SSRIs). SSRIs block the reuptake of serotonin through the serotonin transporter, which then increases serotonin at the synaptic cleft. Serotonin then binds to 5-HT1A receptors, which are G-protein coupled receptors that are present both presynaptically and postsynaptically. Presynaptically, these receptors act as autoinhibitory receptors, triggering decreased firing rates and serotonin release. This autoinhibition, which lasts for about two weeks, is believed to be the reason why patients experience a delay in symptomatic improvement after initiation of SSRIs. Serotonin binding to postsynaptic receptors mediates symptomatic improvement of depression. Multiple positron emission tomography (PET) studies utilizing antagonists at 5-HT1A have been conducted. The results are mixed, with some noting increased and others noting decreased 5-HT1A in the brains of patients with MDD. While clinical heterogeneity and the effects of previous treatment with SSRIs may be confounding factors, the lack of consistent finding could also be secondary to using antagonist rather than agonist radioligands. Unlike agonists, antagonists are unable to discriminate between 5-HT1A in the high and low affinity states, and only the high affinity state, which is G-protein-coupled, allows for activity at the 5-HT1A receptor.